This invention relates to the cubic form of boron nitride and its formation or transformation from the hexagonal form of boron nitride. More particularly, this invention relates to the production of polycrystalline CBN abrasive particles utilizing large-sized HBN powders of an ideal structure and the abrasive tools made from these CBN abrasive particles. The processes used in this invention involve the subjection of large particle boron nitride in the hexagonal form to high pressures and temperatures, either in the absence or presence of catalyst to form cubic boron nitride. The CBN abrasive obtained has an increased grain size and a grain size with a higher aspect ratio, which enhances the performance of abrasive tools made therefrom.
Three crystalline forms of boron nitride are known: (1) hexagonal boron nitride (HBN), a soft graphitic form similar in structure to graphite carbon; (2) wurtzitic boron nitride (WBN), a hard hexagonal form similar to hexagonal diamond; and (3) cubic boron nitride (CBN), a hard zinc blend form similar to cubic diamond. The three boron nitride crystal structures may be visualized as formed by the stacking of a series of sheets or layers of atoms. FIGS. 1-a through 1-c of U.S. Pat. No. 4,188,194 illustrate these three structures in greater detail. In HBN crystals, the boron and nitride atoms bonded together are in the same plane as stacked layers. In the more dense CBN crystal structures, the atoms of the stacked layers are puckered out of plane. In addition, the layers are stacked along the [001] direction in HBN crystals, whereas in the CBN crystal, the layers are stacked along the [111] direction. Furthermore, bonding between the atoms within the layers of an HBN crystal is predominantly of the strong covalent type, with only weak Van derWaals bonding between layers. In CBN crystals, strong, predominantly covalent tetrahedral bonds are formed between each atom and its four neighbors.
Methods for converting HBN into CBN monocrystalline and polycrystalline particles are well known. U.S. Pat. No. 2,947,617 describes a method for preparing cubic boron nitride by the subjection of a hexagonal form of boron nitride, in the presence of a specific additive material, to very high pressures and temperatures. The pressures and temperatures are within the cubic boron nitride stable region defined by the phase diagram of boron nitride. Cubic boron nitride is recovered after removal of the high-pressure and high-temperature condition. The added material or catalyst is selected from the class of alkali metals, alkaline earth metals, tin, lead, antimony and nitrides of these metals. The cubic boron nitride stable region is that represented in FIG. 1 of U.S. Pat. No. 2,947,617 shown above the equilibrium line on the phase diagram therein.
A method for converting HBN to CBN in the absence of catalysts is described in U.S. Pat. No. 3,212,852 under conditions of higher pressures and temperatures. See also: Wakatsuki et al., "Synthesis of Polycrystalline Cubic BN (VI) ," and Ichinose et al., "Synthesis of Polycrystalline Cubic BN (V) ," both in Proceedings of the Fourth International Conference of High Pressure, Kyoto, Japan (1974), pp. 436-445; U.S. Pat. No. 4,016,244; Wakatsuki et al., Japanese Patent No. Sho 49-27518; Wakatsuki et al., Japanese Patent No. Sho 49-30357; Wakatsuki et al., Japanese Patent No. Sho 49-22925; Wakatsuki et al., U.S. Pat. No. 3,852,078; Wakatsuki et al., "Synthesis of Polycrystalline Cubic Boron Nitride," Mat. Res. Bull. 7,999-1004 (1972); and Sirota, N., British Patent No. 1,317,716. Such methods are referred to as direct conversion processes.
In all of these processes, hexagonal boron nitride in powder form is used as a starting material. Two forms of hexagonal boron nitride have been identified, the turbostratic structure and the ideal structure. It has been found that the use of large particle ideal structure HBN powders, having an average particle size of about 10 .mu.m or above, improves the packing density of the cell used in high-pressure/high-temperature equipment, thereby improving the yield of CBN. This process is the subject of copending U.S. application Ser. No. 07/735,503, filed Jul. 25, 1991.
It is the cubic form of boron nitride which finds use as an abrasive material typically in the form of a compact such as a cluster compact or a composite compact, or as particles agglomerated together or bonded to a tool body to form an abrasive tool. In a compact, the abrasive crystals are chemically bonded together, typically in a self-bonded relationship. Individual cubic boron nitride particles are physically bonded together in a metal, resin, or vitrified matrix, such as nickel or phenolic resin, to form an abrasive tool such as a grinding wheel. The individual CBN abrasive particles may also be coated with metals such as Ni, Co, Cu and Ti; intermetallics such as Ni--Al and Ni--B; and ceramic composites prior to incorporation into the abrasive tool. Abrasive tools may also be provided by bonding the cubic boron nitride abrasives directly to the surface of a tool body by conventional electroplating techniques, preferably following preparation of the CBN surface with conventional pretreatments. U.S. Pat. Nos. 3,136,615 and 3,233,988 provide a detailed description of certain types of cluster compacts and methods for their manufacture. U.S. Pat. Nos. 3,743,489 and 3,767,371 provide a detailed disclosure of certain types of composite compacts and methods for their manufacture. U.S. Pat. Nos. 3,081,161; 2,137,200; 2,334,048; and 4,549,372 describe examples of abrasive tools comprised agglomerated particles and methods for their manufacture.
The performance of abrasive tools is often quantified by a grinding ratio, which is the ratio of the amount of material removed from a test specimen to the amount of tool lost. Therefore, a high grinding ratio is indicative of good wear performance. The wear performance of abrasive tools is affected by the cutting surface profile and fracture characteristics provided by the abrasive and its retention in the abrasive tool. The profile of the cutting surface is limited by the retention strength of abrasives in the abrasive tool. It is desirable to provide abrasives with characteristics which will enhance the wear performance of abrasive tools and reduce the energy requirements.